Anatomy Osteology Pdf

[Irmgard Rossie]

Bones are dynamic structures that are undergoing constant change and remodeling in response to the ever-changing environment. [3] In fact, there is so much turnover that in 4 years, the skeleton of a young person will be completely new as compared with their skeleton today. [1] Bones can react and respond to environmental stimuli; they can get bigger or smaller, they can strengthen themselves when needed, and, when broken, they are among the few organs with the ability to regenerate without scar. [1]

There are 206 bones (some say 213 [4] ) in the human body. Some variation exists, because humans may have different numbers of certain bones (eg, vertebrae and ribs). Bones vary widely in size, ranging from the tiny inner ear bones that are responsible for transmitting mechanical sound waves to the sensory organs to the large (nearly 2 ft long) femur bone that is strong enough to withstand 30 times one's body weight.

Long bones are formed from a cartilage model precursor by endochondral ossification (see the image below) and can range in size from a phalanx to a femur. They are typically tubular, have distinct anatomic zones, and are longer than they are wide. [1, 2, 4] Short bones arise from the same precursors but are not necessarily structurally similar to long bones, often taking on unique shapes (eg, carpal bones). Flat bones are formed without a precursor by intramembranous ossification [1] and can have unusual shapes (eg, skull or sternum).

Most bones have a thick, well-organized outer shell (cortex) and a less dense mesh of bony struts in the center (trabecular bone) (see the image below). The ratio of cortical bone to trabecular bone varies widely; [5] in adults, this ratio is typically 80:20. [4]

The only bones that lack a true cortex are the vertebrae, which are covered by a compact condensation of trabecular bone. [1] All bones are encased in a soft tissue envelope known as the periosteum, which is vital for perfusion and nutrient supply to the outer third of the bone (see the image below). [1, 2] The remainder of the blood supply is through nutrient vessels that pierce the cortex and supply the marrow cavity and the inner two thirds of the cortical bone. [1, 2]

Mature long bones have 3 distinct zones: epiphysis, metaphysis, and diaphysis (see the image below). [1] In development, the epiphysis and metaphysis are separated by a fourth zone, known as the epiphyseal plate, or physis. This segment of the bone is cartilaginous and is the region from which the bone grows longitudinally. By adulthood, all epiphyseal plates have closed down, and a bony scar is all that remains of this important structure. Long bones include the femur, tibia, fibula, humerus, radius, ulna, metacarpals, metatarsals, and phalanges.

The epiphysis is the region at the polar ends of long bones. Most commonly associated with joint surfaces, it usually comprises a thin, compact bone shell with a large amount of bony struts (trabecular bone) for support of the cortical shell. The network of bony struts below the compact shell is ideally suited for its job as a shock absorber. [1]

The shell or covering of compact bone is thicker just below a joint and is known as the subchondral bone; it supports the hyaline articular cartilage of the joint just above it. The subchondral bone is not true cortical bone, in that it lacks some of the organization of cortical bone. [1]

The epiphysis also serves as an attachment region in many bones, allowing joint capsular attachments, many ligamentous attachments, and some tendinous attachments as well. Like most sections of bone, it is strong, but it lacks the rigidity of the diaphysis.

There is a layer of resting cartilage that is the precursor to the process. Cells are stimulated to replicate in the zone of proliferation, and chondrocytes then hypertrophy in the zone of hypertrophy. They then undergo a process of mineralization, and eventually death, in the zone of calcification. This forms the bone precursor that will continuously be remodeled throughout life. Bones can also grow in width from direct bone formation supported by the periosteum.

The metaphysis is a transitional zone between the epiphysis and diaphysis. It is also characterized by thinner cortical walls with dense trabecular bone. It is commonly the site of tendinous attachments to bone. It is a metabolically active region and often supports a fair amount of bone marrow. The metaphysis is the region where the bone made by the epiphyseal plate is fine-tuned into its diaphyseal shape.

In the middle of long bones is the diaphysis, a segment of thick cortical bone with a minimal amount of trabecular bone. It is often smaller in diameter than metaphyseal and epiphyseal bone; because its thick cortical layer is extremely strong, it does not require a large diameter to distribute its load. The central portion is the least dense area of the bone and is known as the intramedullary canal. The area of the bone inside the cortex is continuous throughout an entire bone and is known as the endosteal area. [1]

Short bones are also formed by the same cartilage precursor model as long bones; however, they tend to have unique shapes and functions. They provide less overall height than long bones. Like long bones, they have a cortical shell on the periphery and a trabecular inner portion. They vary in size and shape. Examples include the carpal bones, vertebrae, patella, and sesamoid bones.

Although similar to the previously mentioned bones in some respects, flat bones differ completely in their embryologic origin. Stemming from mesenchymal tissue sheets, flat bones never go through a cartilaginous model. The mesenchymal sheets condense and organize and are eventually ossified. They grow from membranous or periosteal growth. They consist of a cortical shell with a cancellous interior and are often broad and flat. They provide protection (eg, skull) and also offer wide, flat surfaces for muscular attachment (eg, scapula).

The skeleton is divided into 2 anatomic regions: axial and appendicular (see the images below). The appendicular skeleton comprises the extremities, which are paired mirror images of each other. The axial skeleton is the central structural core of the body. The auditory ossicles and the hyoid bone are nonstructural, nonextremity bones that are used in sensation, phonation, and swallowing; they do not fit well into either category.

The axial skeleton includes the bones of the skull, cervical vertebrae, thoracic vertebrae, ribs, sternum, [1] lumbar vertebrae, [5] and the sacrum and coccyx (see the image below). Some authors consider the bones of the pelvis to be axial, although they properly belong to the appendicular skeleton.

The cervical spine is made up of 7 vertebrae (see the first and second images below). C1 and C2 are highly specialized and are given unique names: atlas and axis, respectively (see the third image below). C1 and C2 form a unique set of articulations that provide a great degree of mobility for the skull. C1 serves as a ring or washer that the skull rests upon the dens or odontoid process of C2. Approximately 50% of flexion extension of the neck happens between the occiput and C1; 50% of the rotation of the neck happens between C1 and C2.

C3-7 are more classic vertebrae, having a body, pedicles, lamina, spinous processes and facet joints. The cervical spine is highly mobile. The other unique feature of cervical vertebrae is that they contain transverse foramina for the vertebral arteries as they travel cephalad, encased in bone at each level.

The thoracic spine is typically made up of 12 vertebrae. These vertebrae also have a body, pedicles, laminae, spinous processes, and facet joints (see the first two images below). Additionally, they have prominent lateral processes that form the articulation with the paired 12 ribs on either side. The 12 vertebrae, 24 ribs, and sternum together form the chest cavity, allowing negative-pressure respiration and providing protection of the chest wall (see the third image below). The thoracic spine is highly immobile.

The lumbar spine is the next mobile segment of the spine, typically consisting of 5 large vertebrae with classic features, including body, pedicles, lamina, spinous processes, facet joints, and lateral processes (see the image below). The lumbar spine is mobile with all articulations, contributing to flexion-extension, bending, and rotation. The lumbar spine allows truncal mobility.

The lumbar spine connects to the sacrum through the L5-S1 articulation (see the images below). The wedge-shaped sacrum is a fused set of sacral vertebrae. Its primary purpose is to transfer the load from the spine to the pelvis. This happens through the extremely strong and immobile sacroiliac joints. The sacrum also houses the sacral nerve roots from the terminal end of the spinal canal. At the end of the sacrum is the coccyx, which is the vestigial remnant of the tail.

The upper extremities are mirrored paired structures. The upper extremity starts at the shoulder girdle and extends to the finger tips. The shoulder girdle consists of the scapula and the clavicle (see the first and second images below). The clavicle is an S-shaped bone that provides a strut on which the shoulder girdle articulates (see the third image below). It originates at the sternoclavicular joint and terminates at the acromioclavicular joint.

The scapula is a multifunctional bone. Its body (the wide and flat medial portion) is the site of origin of the rotator cuff muscles. Additionally, the scapula articulates with the chest wall to give the shoulder a greater net motion that could be achieved with just glenohumeral motion. The body of the scapula then turns into the neck and flattens into the shallow glenoid cavity.

The glenoid cavity is the socket of the ball-and-socket joint of the shoulder (the glenohumeral joint). It is a deficient socket, being very flat. Accordingly, the soft tissue labrum, ligaments, and muscular attachments are crucial in stabilizing this joint.

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